1. Field of the Invention
Invention relates to turbocharged piston engines. More specifically invention relates to a method of operating turbocharged piston engine comprising the steps of running the engine during a period, shutting down the engine and restarting the engine.
2. Description of Related Art
In internal combustion engine technology turbochargers, in which the energy of the exhaust gas provides the power for running the compressor, are commonly used for increasing the efficiency of the engine operation. A turbo compressor comprises a compressor section, by which pressurized combustion air is introduced into the engine. In addition, the turbo compressor comprises a turbine section for driving the compressor. The exhaust gases coming from the engine are led to the turbine, which converts the energy of the exhaust gases into driving power for the compressor. Forming of crust, such as carbon deposit, has been noticed as a problem in connection with turbine part of turbo compressors. Forming of crust is especially problematic in the blades of the turbine and also clearance between the blades of the turbine and the turbine housing and especially when heavy fuel oil is used as a fuel in a turbocharged piston engine. The surface temperature of the surfaces limiting the gas rises to a high level and the crust forming on the surfaces is especially hard. The properties of the crust vary according to the composition of the fuel; with vanadium- and sodium-containing heavy fuel oils the problem is especially considerable.
In publication EP 1 577 520 A2 there is shown a turbo compressor arrangement in connection with a piston engine, which arrangement comprising a compressor unit and a turbine unit connected to each other, wherein the turbine unit comprises a turbine wheel having blades and a turbine housing enclosing the turbine wheel and the blades at a distance therefrom. There is a cooling space for a cooling medium arranged substantially adjacent the tips of the turbine blades for cooling the turbine housing and avoiding the forming of crust. This arrangement provides a cooling of a limited area of the turbine housing aiming to prevent the formation of the crust. However, the publication does not concern cleaning of already fouled turbine.
Several solutions have been developed for removing carbon deposits from the turbine surfaces, while the turbine remains in operation. Publication U.S. Pat. No. 5,944,483 A shows an apparatus for wet cleaning a nozzle ring of an exhaust-gas turbocharger turbine utilizing a so-called thermal shock method. The method for wet cleaning of the nozzle rings of exhaust-gas turbocharger turbines is based on thermal shock of the contaminants, and includes the steps of injecting water in repeated, relatively small amounts, into the exhaust duct immediately upstream of the nozzle ring. A delay between injections allows the nozzle ring to reheat to operating temperature so that each water injection causes a thermal shock. An apparatus to perform the method includes water injection nozzles installed in the exhaust gas casing and a control system. This method requires the engine to be running while the cleaning is performed. Additionally, using thermal shock for detaching the crust causes also extra stress and load to the structure of the turbocharger.
Other methods of removing carbon deposits from the turbine surfaces are continuous water injection at low load of engine. This takes place in unfavourable conditions, that is, conditions that itself cause most of the contamination. Dry cleaning using hard particles is also known.
US 2008236150 shows a piston engine provided with a turbo compressor which may be operated at a higher output during the cleaning of the turbine. This is accomplished by a method of operating a piston engine that comprises a turbo compressor including a compressor and an exhaust gas turbine coupled to drive the compressor, in which method exhaust gas is cooled by adding air and/or water to the exhaust gas upstream of the turbine while the cleansing agent is fed to the turbine.
WO 2005/049972 A1 discloses a cleaning device for the nozzle ring and the rotating blades of an axial turbine of a turbo compressor. The cleaning device comprises a rod-shaped nozzle protruding into the flow channel upstream the turbine. A cleaning medium is injected into the gas flow through the injection openings of the nozzle. It is also suggested that the nozzle may be advantageously attached to an extra waste gate flange which usually exists due to the fact that the flow channel is often provided with two opposite waste gate openings for facilitating waste gate installation on either side of the flow channel.
Cleaning the turbine during operation of the engine by injecting, for example, water into the exhaust gas stream upstream the turbine causes the detached crust to easily be entrained by the gases to the exhaust gas ducting and to the atmosphere which is not desired.
An aim of the invention is to produce a turbo compressor arrangement in connection with a piston engine by means of which the problems associated with prior art can be minimized.